Combustor exit duct for gas turbine engines

ABSTRACT

A gas turbine engine combustor includes an exit duct having annular first and second exit duct walls radially spaced apart to define therebetween the combustor exit opening. The first and/or second exit duct walls has a double-skin wall section which includes an inner hot wall facing the combustor exit opening and an outer cold wall fastened to the inner hot wall and radially spaced away therefrom to define a radial gap therebetween. The outer cold wall has a coefficient of thermal expansion greater than that of the inner hot wall. This helps reduce thermal growth mismatch between the outer cold wall and the inner hot wall during operation of the combustor, and reduces thermal stress at the joint between the hot and cold walls.

TECHNICAL FIELD

The present invention relates generally to gas turbine engines, and more particularly to combustors for such engines.

BACKGROUND

Combustor performance directly impacts the overall pollutant emission and performance of a gas turbine engine. While cooling air is typically required to cool hot surfaces of combustor liners, the introduction of such cooling air into the main gas path dilutes the hot combustion gas flowing to the turbine and thus reduces the combustor performance. Combustor to turbine transition ducts, particularly in reverse flow combustor configurations, allow the combustion products to have a longer distance to mix with the cool dilution air before striking the turbine blades. This extra mixing length reduces the combustion maximum gas exit temperature. With the help of such longer combustor to turbine transition ducts, lower peak combustor gas exit temperature can be achieved with less dilution flow, so that more can be used for cooling, carbon formation and emission control. The fuel nozzle count may also be able to be reduced, which reduces weight and costs of the combustion system. Also, such reverse flow transitions ducts which feed the combustion gases into the turbine shorten the overall length of the combustor and thus the engine, which greatly reduces weight, cost and simplifies shaft/bearing design. However, these transition ducts are extra areas that need to be cooled and therefore an effective combustor wall construction and/or cooling system thereof is required.

SUMMARY OF THE INVENTION

There is provided a gas turbine engine combustor comprising outer and inner annular liners and an exit duct at a downstream end, the exit duct circumscribing an annular combustor exit opening defining a combustion gas path therethrough, the exit duct including annular first and second exit duct walls radially spaced apart to define therebetween the combustor exit opening, at least one of the first and second exit duct walls having a double-skin wall section at a most downstream end thereof, the double-skin wall section including an inner hot wall facing said combustor exit opening and an outer cold wall radially spaced away from the hot wall to define a radial gap therebetween, the outer cold wall being disposed outside of said combustion gas path, the inner hot wall and the outer cold wall being fastened together by at least one joint therebetween, the outer cold wall having a coefficient of thermal expansion greater than that of the inner hot wall to reducing thermal growth mismatch between the outer cold wall and the inner hot wall during operation of the combustor and reduce thermal stress at the joint.

There is also provided a combustor exit duct for a gas turbine engine, the combustor exit duct comprising annular first and second exit duct walls radially spaced apart to define therebetween a combustor exit opening, at least one of the first and second exit duct walls having a double-skin wall section at a most downstream end thereof, the double-skin wall section including an inner hot wall facing said combustor exit opening and an outer cold wall radially spaced away from the hot wall to define a radial gap therebetween, the outer cold wall being disposed outside of said combustion gas path, the inner hot wall and the outer cold wall being fastened together by at least one joint therebetween, the outer cold wall having a coefficient of thermal expansion greater than that of the inner hot wall to reducing thermal growth mismatch between the outer cold wall and the inner hot wall during operation of the combustor and reduce thermal stress at the joint.

There is further provided a method of forming a gas turbine engine combustor, the combustor having outer and inner annular liners and an exit duct at a downstream end, the method comprising: providing a first and a second annular wall of the exit duct which circumscribe an annular combustor exit opening defining a combustion gas path therethrough; forming a double-skin wall section on at least one of the first and second annular walls of the exit duct, by welding an annular outer cold wall flange to an inner hot wall portion facing said combustor exit opening to form an annular welded joint therebetween, the annular outer cold wall flange being spaced apart from the inner hot wall downstream of said welded joint to define a radial gap therebetween at a downstream end of the double-skin wall section; and reducing thermal stress at the welded joint between the outer cold wall flange and the inner hot wall of the double-skin wall section by forming the outer cold wall flange from a material having a coefficient of thermal expansion that is greater than that of the inner hot wall to thereby reducing thermal growth mismatch between the outer cold wall flange and the inner hot wall.

Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures depicting aspects of the present invention, in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine, partly fragmented, to show the location of the combustor and its exit duct;

FIG. 2 is a fragmented cross-section view showing a portion of the annular exit duct;

FIG. 3 is an enlarged view showing the construction of the double-skin inner annular curved wall of the exit duct; and

FIG. 4 is a cross-section view of the outer annular curved wall of the exit duct.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and more particularly to FIG. 1, there is schematically illustrated a gas turbine engine 10, which is a turbofan in the depicted embodiment but may also be other types of gas turbine engines, preferably adapted for use in an aircraft and subsonic flight. The gas turbine engine 10 generally comprises, in serial flow communication, a fan 12 through which ambient air is propelled, a multi-stage compressor 14 which pressurizes the air from the fan 12 and feeds it towards a combustor 16 in which the compressed air is mixed with fuel and ignited for generating an annular pressurized stream of combustion gases which exits from a combustor exit duct 16′ into a turbine section 18 having turbine rotors 17 and 17′ for extracting energy from the combustion gases.

The combustor 16, and more particularly the combustor exit duct 16′ thereof, will now be described in further detail. As described above, it is desirable to keep cool the combustor-to-turbine transition duct portions but preferably without introducing undue amounts of diluting cooling air flow into the main combustion gas stream.

Accordingly, the combustor exit duct 16′ of the presently described combustor 16 includes a double-skin exit duct wall configuration, which uses the coolant air flow twice, firstly by impingement cooling and secondly by film cooling of the downstream turbine section. As best seen in FIGS. 3 and 4, at least the cold outer walls 24 and 37 of the short exit duct 29 and the long exit duct 20 respectively therefore have impingement cooling holes 40 therein, which direct impingement cooling air flow through the cold outer walls 24, 37 and onto the outer surface of the hotter inner walls 23, 19. Once this cooling airflow has impinged on the outer surfaces of the hotter inner walls 23, 19, it then flows downstream and joins the main combustion flow gas path at the outlet of the combustor exit duct 16′ as film cooling airflow 42. Thus, this same airflow acts to film cool the downstream surfaces, such as the turbine vane platforms. In an alternate embodiment, one or both of the walls or skins which make up the double-skin duct as described herein may therefore have a plurality of cooling air holes therein, such as film cooling holes, impingement cooling holes, effusion cooling holes, etc.

This impingement/film cooling combination using a double-skin duct configuration, however, requires the cooler outer skin to be attached, such as by welding, to the hotter inner skin adjacent the exit end of the combustor exit duct. If left unchecked, this can lead to a thermal fight between the two skins due to the different temperatures to which they are exposed, thereby producing high thermal stresses at the joint (weld, braze, fastening point, etc) interconnecting the two skins. This can cause a reduced fatigue life. Accordingly, the present combustor duct configuration, as will be described in further detail below, provides a design which reduces the high thermal stress at the weld junction between the two skins of the double-skin configuration. By reducing the thermal fight between the double-skins, the fatigue life of the weld junction can be greatly increased.

The combustor exit duct 16′ as described herein includes at least one double-skin wall section which is formed of two formable metal sheets having different coefficients of expansion, thereby greatly reduce the thermal fight between the two skins of the double-skin wall section and thus reducing the thermal stress at the weld joint between these two skins. This combination of welded skins having respective low and high coefficients of expansion allows the designer to have the options of (a) higher fatigue life due to lower stress resulting from the lower thermal fight or for the same fatigue life, (b) higher metal temperature gradient between the two skins for the same fatigue life which results in lower cooling flow and higher combustion performance, (c) thinner skin for less weight and cost or (d) less expensive material.

Referring now to FIGS. 2 to 4, the combustor exit duct 16′ includes generally annular first and second exit duct walls which are radially spaced apart to circumscribe and thus define therebetween the annular exit opening 33 of the combustor 16. In the depicted embodiment, wherein the combustor 16 is a reverse flow combustor, the first and second exit duct walls of the combustor exit duct 16′ respectively comprise a Large Exit Duct (LED) portion 20 and a Small Exit Duct (SED) portion 23. The combustion gases leave the combustor 16 via the annular exit end 33 and are fed into the turbine section 18 disposed downstream therefrom. The LED 20 includes a curved annular wall 19 which is integrally formed at an upstream end thereof with an outer wall 21 of the outer combustor liner 22. Similarly, the SED 29 includes an annular curved wall 23 which is integrally formed at an upstream end thereof with an inner wall 25 of the inner combustor liner 15.

At least one of the first and second exit duct walls, in this embodiment the LED portion 20 and the SED portion 23, have a double-skin wall section at a most downstream end thereof. As will be seen, the double-skin wall section includes inner hot walls 19, 23 facing the combustor exit opening 33 and outer cold walls 37, 24 radially spaced away from the hot wall to define a radial gap 36, 32 therebetween. The outer cold walls 37, 24 are disposed outside of the combustion gas path flowing through the combustor exit opening 33, and are thus exposed to lower temperatures during operation of the engine. The inner hot walls and the outer cold walls are fastened together by at least one joint therebetween, which may include an annular welded joint for example.

As can be see in FIGS. 2 and 3, the SED 29 includes a double-skin wall section comprising an annular inner hot wall 23 facing the combustion chamber 28 and a spaced apart annular outer cold wall 24, in the form of an annular flange wall, at least a portion of which is radially spaced apart from the inner hot wall 23 at the downstream end thereof proximate the exit end opening 33 of the combustor exit duct 16′. An air gap or air plenum 32 is defined between the inner hot wall 23 and the annular flange forming the outer cold wall 24. The annular flange of the outer cold wall 24 is fastened to the inner hot wall 23 of the SED 29 by at least one joint 27. This joint between the inner hot wall 23 and the outer cold wall 24 may be a welded or brazed joint, however other fastening joints may also be possible. In at least the depicted embodiment of FIG. 2, the joint 27 is disposed upstream of the exit end 33 of the combustor exit duct 16′ in a transition area 28.

Similarly, as best seen in FIGS. 2 and 4, the LED 20 includes an outer cold wall 37 at least a portion of which is radially spaced apart from the inner hot wall 19 of the LED 20 at the downstream end thereof proximate the exit 33 of the combustor exit duct 16′, such as to such as to define a radial air gap or air plenum 36 between the hot inner wall 19 and the outer cold wall 37 of the LED 20. While in the embodiment described herein both the LED 20 and the SED 29 are formed having such a double-wall or double-skin construction, it is to be understood that in an alternate embodiment, only one of these portions of the combustor exit duct 16′ may have such a construction (i.e. the other may be simply formed of single liner wall).

Referring back to FIGS. 2 and 3, the annular flange of the cold outer wall 24 of the SED 29 is, in at least one possible embodiment, composed of a formable sheet metal of a type compatible for joining with the sheet metal of the inner hot wall 23, such that a high strength weld joint 27 can be formed therebetween near the upstream transition area 28 as shown.

As the inner hot wall 23 is directed exposed to the hot combustion gases within the combustor 16, it is subjected to higher temperatures than the outer cold wall 24 which is both spaced apart from the hotter inner hot wall 23 of the SED 29 and also exposed to more cooling air disposed around the combustor. Accordingly, in order to compensate for this temperature difference, the outer cold wall 24 has a coefficient of thermal expansion that is higher than the coefficient of thermal expansion of the inner hot wall 23 of the SED 29. This acts to reduce the thermal imbalance in the double skin wall which forms the SED 29 at the exit end 33 of the combustor exit duct 16′. In other words, as the inner hot wall 23 heats up during operation of the combustor 16, the difference in the coefficients of thermal expansion between the inner hot wall 23 and the outer cold wall 24 will result in both walls, or skins, expanding at approximately the same rate and approximately the same amount. Thus, the thermal growth of the two “skins” of the double-skin SED 29 are more closely matched, as a result of this mismatch in the coefficients of thermal expansion of the two walls 23 and 24.

This difference in thermal expansion may be achieved, for example, by forming the inner and outer walls, or skins, 23 and 24 of the SED 29 out of different materials, such as two different sheet metals for example, each having a different coefficient of thermal expansion (i.e. the outer wall 24 expanding more at a given temperature than the inner wall 23). Alternately, this difference in thermal expansion between the two walls 23, 24 may be achieved by other means, rather than by having different coefficients of thermal expansion, for example by making the two walls of different thickness or different material properties such as to achieve a similar thermal growth match during operation of the engine, when the inner wall 23 of the SED 29 is exposed to higher temperatures than the outer wall 24 thereof.

In at least the depicted embodiment, the outer cold wall 24 is formed as an annular flange having a ring shape skin with an outer flat portion 30 positioned against a planar section of the combustor inner annular wall, where the weld joint 27 is formed. The outer wall 24 also has a curved portion 31 which has a radius of curvature different from the radius of curvature of the inner wall 23 to form the annular gap or plenum 32 therebetween.

It at least one embodiment, the LED 20 is similarly constructed with a double-skin wall section, as per the SED 29 described above. Referring now to FIGS. 2 and 4, as described above the LED 20 includes an outer cold wall 37 at least a portion of which is spaced apart from the curved annular inner hot wall 19 of the LED 20 at the downstream end thereof proximate the exit 33 of the combustor exit duct 16′, such as to such as to define an air gap or air plenum 36 between the annular wall 19 and the annular flange wall 37 on a cold side of the wall 19 of the LED 20. The outer cold wall 37 is joined to the main combustor wall which forms the inner hot wall 19, such as by a welded joint for example, at joint 38. The welded joint 38 may be disposed about the outer surface 20′ of the outer annular liner 21 and spaced from the exit end 33. Much as per the SED described above, the outer cold wall 37 of the LED 20 has a coefficient of expansion which is higher than the coefficient of expansion of the inner hot wall 19, such as to reduce thermal growth mismatch in the double-skin wall arrangement during operation of the combustor, and thus reducing the thermal stress to which the welded joint between the hot and cold walls is exposed.

The outer cold wall 37 is also formed from a formable metal sheet of a type compatible for fastening, such as by a weld or brace, to the inner hot wall 19. The outer cold wall 37 has an outer portion 26 for connection to the outer liner 21 and an outwardly offset wall section 37′. The radially extending annular gap 36 is formed between an end section of said outer liner 21 and the outwardly offset wall section 37′.

In one particular embodiment, the outer cold walls 24 and 37 may be formed from a Hastalloy X (registered trademark) sheet metal, and the hot inner walls 19 and 23 are formed from one of 1N625, Haynes 188 and Haynes 230 sheet metals.

In an exemplary operating environment of the combustor 16, the hot annular walls 19 and 23, of the LED 20 and the SED 29 respectively, which are directly exposed to the hot combustion gases, are subjected to temperatures of at least 1650° F. The outer cold walls 37 and 24, of the LED 20 and the SED 29 respectively, are disposed in relatively cooler areas surrounding the combustor, outside the main gas path, and are thus subjected to lower temperatures of at least 800-1500° F. This difference in temperature would typically cause, in a double-wall construction wherein the two skins are the same material, the hotter inner walls to expand more than the cooler outer walls. In the combustor exit duct 16′ of the present combustor 16, the aforementioned differential in the coefficients of thermal expansion between the hot and the cold walls of the double skin liner construction results in the two walls expanding approximately a similar amount, thereby substantially compensating for the differential in temperature during operation of the combustor. Accordingly, this reduction of the thermal growth between the two walls results in less stress being placed on the welded joints 27 and 38 between the hot and cold walls of the double-wall construction.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims. 

1. A gas turbine engine combustor comprising outer and inner annular liners and an exit duct at a downstream end, the exit duct circumscribing an annular combustor exit opening defining a combustion gas path therethrough, the exit duct including annular first and second exit duct walls radially spaced apart to define therebetween the combustor exit opening, at least one of the first and second exit duct walls having a double-skin wall section at a most downstream end thereof, the double-skin wall section including an inner hot wall facing said combustor exit opening and an outer cold wall radially spaced away from the hot wall to define a radial gap therebetween, the outer cold wall being disposed outside of said combustion gas path, the inner hot wall and the outer cold wall being fastened together by at least one joint therebetween, the outer cold wall having a coefficient of thermal expansion greater than that of the inner hot wall to reducing thermal growth mismatch between the outer cold wall and the inner hot wall during operation of the combustor and reduce thermal stress at the joint.
 2. The gas turbine engine combustor as defined in claim 1, wherein the combustor is a reverse flow combustor, the first and second exit duct walls respectively comprising a large exit duct portion and a small exit duct portion.
 3. The gas turbine engine combustor as defined in claim 2, wherein both the large exit duct portion and the small exit duct portion comprise said double-skin wall section at the most downstream ends thereof.
 4. The gas turbine engine combustor as defined in claim 3, wherein the inner hot wall and the outer cold wall of the short exit duct comprise spaced apart curved wall portions, a radius of curvature of the inner hot wall being different from that of the outer cold wall.
 5. The gas turbine engine combustor as defined in claim 1, wherein the joint between the inner hot wall and the outer cold wall includes a welded joint disposed upstream of an exit end of the double-skin wall section.
 6. The gas turbine engine combustor as defined in claim 5, wherein said welded joint is disposed at an upstream end of the combustor exit duct in a transition area of the combustor disposed between the outer and inner annular liners and the exit duct.
 7. The gas turbine engine combustor as defined in claim 1, wherein the outer cold wall of the double-skin wall section includes an annular flange wall being formed from a formable metal sheet of a type compatible for welding to the inner hot wall.
 8. The gas turbine engine combustor as defined in claim 1, wherein both the inner hot wall and the outer cold wall are made of sheet metal.
 9. The gas turbine engine combustor as defined in claim 1, wherein outer cold wall is formed of Hastalloy X™.
 10. The gas turbine engine combustor as defined in claim 1, wherein the inner hot wall is formed of one of IN625, Haynes 188 and Haynes
 230. 11. The gas turbine engine combustor as defined in claim 1, wherein at least the outer cold wall of said double-skin wall section includes one or more impingement cooling holes therein.
 12. A combustor exit duct for a gas turbine engine, the combustor exit duct comprising annular first and second exit duct walls radially spaced apart to define therebetween a combustor exit opening, at least one of the first and second exit duct walls having a double-skin wall section at a most downstream end thereof, the double-skin wall section including an inner hot wall facing said combustor exit opening and an outer cold wall radially spaced away from the hot wall to define a radial gap therebetween, the outer cold wall being disposed outside of said combustion gas path, the inner hot wall and the outer cold wall being fastened together by at least one joint therebetween, the outer cold wall having a coefficient of thermal expansion greater than that of the inner hot wall to reducing thermal growth mismatch between the outer cold wall and the inner hot wall during operation of the combustor and reduce thermal stress at the joint.
 13. The combustor exit duct as defined in claim 12, wherein both the first and second exit duct walls comprise said double-skin wall section at the most downstream ends thereof.
 14. The combustor exit duct as defined in claim 12, wherein the joint between the inner hot wall and the outer cold wall includes a welded joint disposed upstream of an exit end of the double-skin wall section.
 15. The combustor exit duct as defined in claim 12, wherein the outer cold wall of the double-skin wall section includes an annular flange formed from a formable metal sheet of a type compatible for welding to the inner hot wall.
 16. The combustor exit duct as defined in claim 12, wherein both the inner hot wall and the outer cold wall are made of sheet metal.
 17. The combustor exit duct as defined in claim 12, wherein outer cold wall is formed of Hastalloy X™, and the inner hot wall is formed from one of IN625, Haynes 188 and Haynes
 230. 18. The combustor exit duct as defined in claim 12, wherein at least the outer cold wall of said double-skin wall section includes one or more impingement cooling holes therein.
 19. A method of forming a gas turbine engine combustor, the combustor having outer and inner annular liners and an exit duct at a downstream end, the method comprising: providing a first and a second annular wall of the exit duct which circumscribe an annular combustor exit opening defining a combustion gas path therethrough; forming a double-skin wall section on at least one of the first and second annular walls of the exit duct, by welding an annular outer cold wall flange to an inner hot wall portion facing said combustor exit opening to form an annular welded joint therebetween, the annular outer cold wall flange being spaced apart from the inner hot wall downstream of said welded joint to define a radial gap therebetween at a downstream end of the double-skin wall section; and reducing thermal stress at the welded joint between the outer cold wall flange and the inner hot wall of the double-skin wall section by forming the outer cold wall flange from a material having a coefficient of thermal expansion that is greater than that of the inner hot wall to thereby reducing thermal growth mismatch between the outer cold wall flange and the inner hot wall.
 20. The method as claimed in claim 19, further comprising forming both the inner hot wall and the outer cold wall flange with a curved portion, the curved portion of the annular outer cold wall flange having a different radius of curvature than that of the inner hot wall. 